The output parameters of many prior art cardiac pacemakers are either preset during fabrication, or established at the time of implant. With such pacemakers, adjustment of any output parameter requires a surgical exposure of the pacemaker itself. Other pacemakers are adjustable through the use of a needle-like tool.
More recently, various systems have been advanced for altering the output parameters of an implanted cardiac pacemaker with transmitted signals of electromagnetic energy. The pacemakers of these systems have included elements responsive to a preselected signal for altering at least one output parameter of the pacemaker on the occurrence of the signal. For example, in U.S. Pat. No. 3,311,111, the use of bistable magnetic reed switches is proposed for the control of pulse rate, voltage, current or duration as well as a selection of alternate output paths or leads. Other systems have been proposed in which pulsed signals are used to advance a counter, the accumulated count in the counter serving to establish the value of the output parameter or parameters to be altered. Pulses in this latter system may be magnetic or bursts of radio frequency energy, for example.
Variations in the "pulsed signal" systems described above have been proposed to reduce the possibility of an alteration in an output parameter of an implanted body stimulator via an extraneous signal. For example, in U.S. Pat. No. 3,805,796 there is disclosed a system having a first counter which advances in response to all detected pulse signals while a second counter is advanced only in response to signals detected after the count of the first counter reaches a preselected value. The value of the count in the second counter is employed to control at least one alterable output parameter. Thus, extraneous signals which are incapable of advancing the first counter to the preselected value cannot result in an alteration of the output parameter of the implanted device. Other systems have been proposed in which a first signal, a magnetic field, for example, enables the implanted device to respond to pulse signals for alteration of an output parameter in predetermined correspondence with the number of pulse signals. An example of such a system is disclosed in co-pending application Ser. No. 584,131 of John M. Adams and Clifton A. Alferness for Programmable Body Stimulator, filed June 5, 1975, which is co-owned with the present application.
From the above, it is apparent that much attention has been directed to providing implantable devices with alterable output parameters and to reducing the probability of an accidental output parameter alteration as a result of extraneous noise. It is contemplated that any desired output parameter alteration will be effected in a doctor's office and, preferably with a portable, non-invasive programming unit. It is therefore desirable that any programming unit have the capability of reliably generating and transmitting the desired number of signals with a minimum drain on the unit power supply. Additionally, it is desirable that such a unit have the ability to indicate the adequacy of the power supply and prevent the transmission in the event that the power supply voltage level is below an adequate level.